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Glossary

Asteroid
Rocky, carbonaceous, or metallic body, smaller than a planet and orbiting the Sun. Most asteroids are in semistable orbits between Mars and Jupiter, but others are thrown onto orbits crossing those of the major planets.
Comet
Body containing a significant fraction of ices, smaller than a planet an dorbiting the Sun, usually in a highly eccentric orbit. Most comets are stored far from the planetary system in two large reservoirs: the Kuiper belt beyond the orbit of Neptune, and the Oort cloud at near-interstellar distances.
“Encyclopedia of the Solar System” Edited by Paul R. Weissman, Luch-Ann McFadden and Torrence V. Johnson, Academic Press 1999.
Eccentricity
Measure of the departure of an orbit from a perfect circle. A circular orbit has an eccentricity of e = 0; an elliptical orbit has 0 < e < 1; a parabolic orbit has e = 1; and a hyperbolic orbit has e > 1.
“Encyclopedia of the Solar System” Edited by Paul R. Weissman, Luch-Ann McFadden and Torrence V. Johnson, Academic Press 1999.
Inclination
Angle between the plane of the orbit of a planet, comet, or asteroid and the ecliptic plane, or between a satellite’s orbit and the equatorial plane of its primary.
“Encyclopedia of the Solar System” Edited by Paul R. Weissman, Luch-Ann McFadden and Torrence V. Johnson, Academic Press 1999.
Kuiper Belt
Collection of some 1-10 billion or more icy bodies in low-eccentricity, low-inclination orbits beyond Neptune, extending out possibly to 1000 AU.
“Encyclopedia of the Solar System” Edited by Paul R. Weissman, Luch-Ann McFadden and Torrence V. Johnson, Academic Press 1999.
Planet
Large body orbiting the Sun or another star, but not large enough to generate energy through nuclear fusion at its core. No formal definition of a planet exists and classifying exactly what is and is not a planet is often quite difficult. Some definitions demand that a planet should have an atmosphere, and/or a satellite, and/or be large enough to form itself into a sphere by self-gravity, and/or be able to gravitationally dominate its region of heliocentric space, but there are counter examples to every one of these requirements.
“Encyclopedia of the Solar System” Edited by Paul R. Weissman, Luch-Ann McFadden and Torrence V. Johnson, Academic Press 1999.
Planetesimal
Small body formed in the early solar system by accretion of dust and ice (if present) in the central plane of the solar nebula.
“Encyclopedia of the Solar System” Edited by Paul R. Weissman, Luch-Ann McFadden and Torrence V. Johnson, Academic Press 1999.
Satellite
Body in orbit around a planet. A satellite was recently discovered orbiting an asteroid and several other satellites are suspected to exist.